The present invention relates to a polishing apparatus and a polishing method capable of performing polishing of a work, for example, a silicon wafer (hereinafter may be simply referred to as xe2x80x9cwaferxe2x80x9d) or the like with high efficiency and high precision, a novel work holding plate for holding a work (for example, a wafer or the like) in a efficient way and a method for adhering a work onto the work holding plate.
Reflecting a tendency to prepare larger diameter silicon wafers and fabricate higher precision devices therewith, requirements for finish precision (thickness uniformity, flatness and smoothness) of a silicon wafer subjected to polishing finish (polished wafer) have been increasingly enhanced.
In order to satisfy such requirements, efforts have been made to attain a higher level in wafer polishing technique, and development and improvement of polishing apparatuses have been carried out.
As one example thereof, so-called single wafer polishing apparatuses have been newly developed for the purpose of polishing a large diameter wafer, especially 300 mm or more in diameter, and some of them have been practically used.
In the single wafer polishing method, however, there arise problems: for example, (1) requirements for reduction in wafer cost is hard to meet in terms of productivity, and (2) recent requirements for wafer flatness as far as an peripheral area adjacent to the wafer edge (within 2 mm) cannot be sufficiently satisfied.
Meanwhile, there has been widely used a batch type polishing apparatus in which a plurality of wafers are simultaneously polished. An outline of a configuration of a portion of the apparatus directly associated with polishing action is shown in FIG. 19. In this polishing apparatus, one or more wafers W are held by means of such as adhesion on a lower surface of a work holding plate 13 rotated by a rotary shaft 18; to-be-polished surfaces of the wafers W are pushed, for example, using a top weight 15 onto a surface of a polishing cloth 16 adhered on an upper surface of a polishing table 10, which is rotated at a prescribed rotational speed by a rotary shaft 17; and a polishing agent solution (hereinafter may be referred to xe2x80x9cslurryxe2x80x9d) 19 is simultaneously supplied at a prescribed rate onto the polishing cloth 16 through a polishing agent supply pipe 14 from a polishing agent supply device (not shown). In such a situation, polishing of the wafers W are performed while the to-be-polished surfaces of the wafers W are rubbed by the surface of the polishing cloth 16 in the presence of the polishing agent solution 19 therebetween.
In this batch type polishing apparatus, there is increasing difficulty in satisfying requirements for precision of finish surfaces of the wafers in a trend of transition to larger-sized apparatuses in company with larger diameter wafers for the following reasons: deflection of a polishing table and work holding plates by weights thereof and polishing pressure, and thermal deformation by heat generation in polishing action; and in addition thereto, deformation and displacement of the polishing table and the work holding plates caused by various kinds of mechanical deflections in rotation thereof.
In order to cope with such problems, various kinds of ingenious contrivances have been practiced about a structure and materials, and operating conditions of the polishing apparatus and other polishing conditions. For example, some of contrivances on the structure are as follows: (a) in order to prevent thermal deformation of a polishing table, as shown in FIG. 20, a separate lower table 23 on which multiple recesses 21 for circulating a cooling water H are formed is provided on a lower surface of an upper table 12 on an upper surface of which the polishing cloth 16 is adhered; further, ribs are provided on a lower surface of a polishing table to prevent deformation due to polishing pressure; and still further in order to effectively suppress thermal deformation, contrivances have been piled up about a structure of a polishing table and arrangement of flow paths of cooling water, as shown in JP-A-95-52034 and JP-A-98-296619.
In a prior art polishing table shown in FIG. 20, however, there is adopted a structure in which an upper table 12 made of SUS410 and a lower table 23 made of cast iron such as FC-30 provided with flow paths for cooling water are coupled to each other by fastening them with clamping members 11 or the like, and a temperature difference between the upper and lower surfaces of the upper table arising in the course of a prior art polishing operation is generally 3xc2x0 C. or higher and, in higher cases, 5xc2x0 C. or higher; therefore, a difference in height (deformation) at a highest or lowest point occurs inconveniently in places on the upper surface of the upper table amounting to 100 xcexcm or more relative to the reference plane, namely the upper surface of the upper table with no temperature difference between the upper and lower surfaces thereof.
Furthermore, the following proposals have been made: (b) that a material with a low thermal expansion coefficient (8xc3x9710xe2x88x926/xc2x0 C.) is used as a material of a polishing table (WO94/13847), that a polishing table is of a one-piece structure made of ceramics in which a flow path for circulating cooling water is formed throughout almost all of the interior (JUM-A-84-151655), and the like techniques; and in addition, (c) that a temperature control fluid is likewise circulated in a work holding plate for the purpose of improving temperature uniformity across a wafer holding surface of the wafer holding plate (JP-A-97-29591).
Moreover, in order to suppress a temperature rise of a wafer and a polishing cloth due to heat generation accompanying polishing action, the following procedures have been performed: in addition to the cooling of the work holding plate and the polishing table described above, a cooling function is also given to a polishing agent solution (in usual case, a weak alkaline aqueous solution mixed with colloidal silica is used.) supplied directly onto a polishing action surface, an amount of the polishing agent solution exceeding a supply amount necessary for polishing action in a pure sense is supplied onto the polishing cloth, and the polishing agent solution discharged from a polishing site is recycled in order to reduce the cost.
In the construction of the prior art polishing apparatus and a cooling method as described above, a temperature on a polishing cloth surface during polishing gradually rises from the start of polishing and a value of the temperature at a portion where the polishing cloth is put in contact with a to-be-polished surface of the wafer rises usually to 10xc2x0 C. or higher and a temperature at a corresponding upper surface portion of the polishing plate direct under the portion of the polishing cloth in the contact also rises by 3xc2x0 C. or more.
On the other hand, changes in temperature on the lower surface of the polishing plate are restricted to 1xc2x0 C. or less by virtue of an effect of suppression of a temperature rise by cooling water. Therefore, a temperature difference of at least 3xc2x0 C. or more arises not only between the upper and lower surfaces of the polishing table, but also between a high temperature portion and a low temperature portion on the upper surface of the polishing table, which causes a portion of the upper surface of the polishing table with thermal deformation/displacement of 100 xcexcm or more in a direction normal to the upper surface of the polishing table in comparison with that when no temperature difference exists.
Furthermore, a work holding plate has become larger in size in response to transition in diameter of a silicon wafer toward a larger value. For example, in case of a work holding plate for use in polishing of 8 inch wafers, a diameter of the work holding plate assumes about 600 mm and a weight thereof also increases as the diameter increases.
Accordingly, not only thermal deformation of a work holding plate caused by heat generation at a polishing surface but also deformation caused by a weight of the work holding plate are problematic; therefore, various trials have been performed in order to suppress such deformation: to increase in thickness of a work holding plate or to decrease deformation by use of a material whose modulus of longitudinal elasticity is large, such as ceramics (silicon carbide and alumina).
Moreover, in a prior art batch polishing, as shown in FIG. 21, for example, a method was adopted in which a to-be-polished wafer W is adhered on a work adhesion surface 20a of the work holding plate 20 with an adhesive 22 applied therebetween.
In this case, it is important that no air bubble is left behind in a adhesive 22 layer and at interfaces between the wafer or the work holding plate 20 and the adhesive 22. For this purpose, a adhering process goes in the following way: as shown in FIG. 21, an air bag 27 expanding so as to be convex downward and provided on a lower surface of a pressure head 25 is pushed onto an upper surface (a surface opposed to the to-be-adhered surface) of the wafer W by the action of a pressure cylinder 26 and a contact surface under pressure of the air bag with the upper surface is increased by the push from the central portion of the to-be-adhered surface of the wafer sequentially part by part toward the periphery thereof such that air in the adhesive and at the adhesion interfaces are driven out and beyond the outer edge of the periphery of the wafer. However, while air in a boundary layer between an adhesive and each of the wafer W and the work holding plate is expelled by such a push-out method with a wafer pressure member 24, a thickness of the adhesive layer 22, on the other hand, is apt to be thinner at a central portion of the wafer W, which causes an inconvenience that the wafer W is fixed in a distorted state.
While, in the prior art, natural rosin, synthetic rosin ester, beeswax, phenol resin and so on were employed as adhesives for use in adhesion of a wafer taking into consideration various factors such as dissolution resistance to a polishing agent solution, a non-lubricating property, a change in characteristics due to a temperature rise of the adhesive through a temperature rise due to polishing heat generation, adhering action by such adhesives is mainly dependent on a physical adhesion mechanism, which goes like this: After an adhesive dissolved in a solvent is applied on an adhesion surface of the wafer holding plate, the solvent is evaporated off, and then, a wafer is pushed onto the work holding plate at a prescribed pressure while keeping the adhesive in a softened state under heat application and thereafter, the adhesive is solidified by cooling to a normal temperature to complete the adhesion.
In such an adhering process, it is necessary to heat a wafer and a work holding plate at a temperature, for example, ranging from 50 to 100xc2x0 C. and improvement on processing precision is retarded by deformation of the wafer and the work holding plate caused by a thermal history in the heat treatment. In addition, there are required special apparatuses and facilities, and energy consumption for such heat treatment and others, which has also become problematic in an aspect of cost.
On the other hand, so-called normal temperature adhesives that have been available, which exert adhering action at normal temperature have not been able to be used in a practical aspect because of weak points such as low dissolution resistance to a polishing agent solution, difficulty in separating a wafer from a work holding plate and difficulty in removing the adhesive from a work holding plate.
Furthermore, in order to prevent air bubbles from being left behind in an adhesive at an adhesion site, the following processes have been practiced: a method in which a to-be-adhered surface of a wafer is pressed onto the work holding plate with an adhesive therebetween while holding the to-be-adhered surface of the wafer so as to be inclined to a work holding surface of the work holding plate and a contact surface is increased by the push from the one edge of the wafer sequentially part by part toward the edge opposite to the one edge thereof such that air in the adhesive between the to-be-adhered surface of the wafer and the work holding surface is expelled from one edge of the to-be-adhered surface of the wafer toward the edge opposite to the one edge thereof, a method in which as shown in FIG. 21, an elastic member having a convex front shape(air bag) 27 is pressed onto the upper surface of the wafer W placed on the work holding plate 20 while increasing a contact area part by part sequentially from the central portion of the wafer toward the periphery of the wafer to expel the air to the outside; and a method in which the whole of the work holding plate 20 or each wafer W is sealed by a holding surface of the work holding plate 20 so as to be air tight and the interior space closed by the sealing is evacuated into a reduced pressure state, whereby no air is left behind.
In FIG. 22, 1 indicates a vacuum vessel; 2, bellows; 3, a cylinder for vertically shifting bellows; 4, an internal pressure adjusting pipe for bellows; 5, an internal pressure adjusting pipe for a vacuum vessel; 6, a vacuum suction pipe; 20, a work holding plate; and W, a wafer.
A fault that a thickness of an adhesive layer becomes non-uniform (equal to or more than 0.5 xcexcm) is problematic in a method shown in FIG. 21 in which a to-be-adhered surface of a wafer is pushed to increase a contact area sequentially part by part from a portion of the to-be-adhered surface thereof, while problems arise such as that a special apparatus and special tools are required and a process is complex, and in addition that dust is generated from the apparatus and tools in a method shown in FIG. 22 in which a wafer or all of a work holding plate is placed in a vacuum state to complete adhesion.
In polishing finish of a wafer, as described above, there have been various factors that are obstacles against achievement of high precision finish thereof meeting higher level of device fabrication techniques now and in the future, not only in connection with deformation by various causes of a polishing apparatus: particularly a work holding plate directly holding a wafer, which is a to-be-processed work, and a polishing table on which a polishing cloth in contact with the wafer is adhered and variations in operation of the apparatus, but also in connection with an adhering method for pasting the wafer on the work holding plate.
The inventors have drastically studied on factors which work as obstacles against high precision processing in connection with not only construction, configuration and materials of a polishing apparatus but also in connection with all the process relating to wafer polishing including an adhering apparatus for a wafer and an adhering method therefor in order to efficiently produce a high precision polishing finish wafer, especially, a high precision wafer of a large diameter of 300 mm or more, in a stable manner through trial manufacture of an apparatus and empirical, comparative studies on a system configuration and operating conditions, with the result that a success has been achieved that a high precision polished wafer can be stably produced by integrally enhancing functions and performance of not only the adhering method for a wafer but also the polishing apparatus and besides, improving an operating method therefor fundamentally.
Among achievements of the above described studies, it has been found that deformation during a polishing operation occurring in a polishing table, on which a polishing cloth is adhered and which is a base for holding a shape of a polishing cloth, or in a work holding plate, which is a base for holding a wafer, is a great obstacle against polishing a high precision (high flatness) wafer and further that it is effective that polishing is performed such that an amount of deformation of the polishing table in a direction normal to an upper surface thereof or an amount of deformation of the work holding plate in a direction normal to a work holding surface thereof is kept to be 100 xcexcm or less, preferably 30 xcexcm or less, more preferably 10 xcexcm or less.
It is accordingly an object of the present invention to provide a polishing apparatus and a polishing method both capable of performing polishing a work (such as a wafer) with high efficiency and high precision, a novel work holding plate effectively holding a work and an adhering method for a wafer capable of adhering the work on the work holding plate with high precision.
In order to solve the above described problems, a first aspect of a polishing apparatus of the present invention comprises: a polishing table; and a work holding plate, wherein a work held on the work holding plate is polished supplying a polishing agent solution, and in polishing action, an amount of deformation of the polishing table in a direction normal to an upper surface thereof and/or an amount of deformation of the work holding plate in a direction normal to a work holding surface thereof is restricted to 100 xcexcm or less. The amount of deformation is preferably restricted to 30 xcexcm or less.
A second aspect of a polishing apparatus of the present invention comprises: a polishing table; and a work holding plate, wherein a work held on the work holding plate is polished supplying a polishing agent solution, and the polishing table is formed in one-piece by casting, a structure of the polishing table is such that a plurality of recesses and/or a plurality of ribs are provided on a rear surface thereof, a flow path for a temperature adjusting fluid is formed inside of the polishing table, and portions in each of which the flow path is not formed act as an internal rib structure.
That is, the polishing apparatus of the present invention has a great feature of the one-piece polishing table which includes a flow path for a temperature adjusting fluid and recesses and/or ribs on a rear surface thereof and also includes the internal rib structure inside thereof, and thereby can enjoy the following advantages:
(1) Comparing with the prior art structure in which an upper table 12 and a lower table 13 illustrated in FIGS. 16 and 17 are fastened with clamping members 11, and a table of a double layer structure disclosed in JP-A-98-296619, the structure of the present invention is higher in strength, and hence can suppress thermal deformation and deformation caused by a pressure of cooling water into a lower level.
(2) It is, therefore, possible to make the polishing table thinner in the total thickness and lighter in the weight.
(3) There arises no secular change such as looseness of the clamping members.
(4) Due to no requirement for clamping sites, it is possible to distribute more widely a flow path for a cooling fluid (for temperature adjustment), enlarge a heat transfer area, reduce a pressure loss along the flow path, and then flow a larger amount of the fluid, thereby a cooling effect being improved by a great margin.
(5) Due to the thinner structure of the polishing table, distances between the surface of the table and a cooling water flow path can be made shorter, thereby a cooling effect being improved more correspondingly to reduction in the distances. Furthermore, in the above structure of the polishing table, displacement of an upper surface of the polishing table relative to a reference plane can be restricted to 100 xcexcm or less at any point thereof, 30 xcexcm or less by further adopting various kinds of structures of the present invention described below, and 10 xcexcm or less in an ideal state.
It is preferable that a value of a thermal expansion coefficient of a material of the polishing table is 5xc3x9710xe2x88x926/xc2x0 C. or less and corrosion resistance of the material is almost equal to that of stainless steel.
As the material of the above described polishing table, when invar, that is, stainless invar which is cast steel, for example, SLE-20A (made by Shinhokoku Steel Corp.) is used, a thermal expansion coefficient (xcex1=2.5xc3x9710xe2x88x926/xc2x0 C., wherein a is a linear expansion coefficient) is about xc2xc as compared with SUS410 (xcex1=1.03xc3x9710xe2x88x925/xc2x0 C.); therefore, an amount of deformation of 30 xcexcm or less can be realized. Furthermore, by thus fabricating a polishing table by casting cast steel, a one-piece structure can be achieved and the following precision processing finish of the polishing table becomes easy.
A third aspect of the present invention comprises: a polishing table; and a work holding plate, wherein a work held on the work holding plate is polished and temperature changes of the polishing table and/or temperature changes of the work holding plate in polishing action are controlled within a prescribed range by controlling a flow rate and/or a temperature of a temperature adjusting fluid.
Temperature changes are preferably within 3xc2x0 C., more preferably within 2xc2x0 C. at any position of the polishing table and/or the work holding plate in polishing action. In order to attain the purpose, as described above, the polishing table of the one-piece structure internally having the temperature adjusting fluid flow path is capable of very effectively increasing a contact area between the temperature adjusting fluid and the polishing table.
Furthermore, temperature changes at any position on a polishing surface of the polishing cloth in polishing action are preferably controlled to 10xc2x0 C. or less, preferably to 5xc2x0 C. or less by controlling a temperature and /or a flow rate of the polishing agent solution.
That is, under ordinary conditions for achieving a prescribed polishing speed (0.5 to 1.0 xcexcm/min) by a prior art polishing apparatus, a temperature on a surface of a polishing cloth rises by heat generation accompanying polishing action and the temperature changes in excess of 10xc2x0 C., especially at a site at which the polishing cloth is rubbed by the to-be-polished surface of the wafer; in order to realize the fundamental concept of the present invention that temperature changes (variations) on the polishing table and/or the work holding plate during polishing action are restricted to within 3xc2x0 C., and an amount of deformation thereof, especially that in a direction normal to an upper surface of the polishing table or a work holding surface of the work holding plate is kept to be 100 xcexcm or less, preferably 30 xcexcm or less, more preferably 10 xcexcm or less, it is important that temperature changes are controlled to 10xc2x0 C. or less, preferably 5xc2x0 C. or less on the surface of the polishing cloth and the wafer of heat generation sites in polishing.
In an actual practice of polishing, as described above, a polishing cloth most suited for the purpose and conditions of polishing is selected and adhered on a upper surface of a polishing table; applying a polishing agent solution between the polishing cloth and a to-be-polished surface of a wafer, the wafer and the polishing cloth are rubbed each other by a relative motion under a prescribed force pressing each other. A thermal conductivity of a polishing cloth generally shows a value lower than those of silicon and material of the polishing table or a work holding plate by one to three orders of magnitudes. Usually, a thickness of a polishing cloth ranges from 1 to 2 mm and a thermal resistance from the front surface of the polishing cloth to the upper surface of the polishing table through the polishing cloth is the greatest, compared with a distance from the upper surface of the polishing table to a temperature adjusting fluid flow path (10 to 50 mm) and a heat transfer distance from a work holding surface of the work holding plate to the temperature adjusting fluid flow path (10 to 30 mm); therefore, if temperature changes on the surface of the polishing cloth in polishing action are restricted to the lowest possible temperature in the range of 10xc2x0 C. or less, preferably 5xc2x0 C. or less, temperature changes at any points on an upper surface of the polishing table or a work holding surface of the work holding plate in the polishing action can be restricted within 3xc2x0 C. and preferably within 2xc2x0 C.
At this time, it is important that cooling effects of the temperature adjusting fluid for the polishing table or the work holding plate are effectively exerted and also necessary that an cooling effect of the polishing agent solution is utilized positively.
In the above description, there are shown important requirements for realizing the fundamental concept of the present invention in connection with the polishing table, the work holding plate and the polishing agent solution, which are members directly associated with polishing action in the polishing apparatus and the operation (polishing) thereof; in order to effectively realize requirements, factors associated with a mechanism and control of the polishing apparatus are also very important. That is, it is necessary that mechanical variations accompanying driving (rotation) of the polishing table and precision of temperature control clear respective prescribed levels, which will be described below in a concrete manner.
Rotational unevenness of the polishing table is preferably restricted to 1% or less. The rotational unevenness of the polishing table means a proportion of variations in rotational speed of the polishing table in polishing action to a preset value thereof.
Surface displacement in rotation of a polishing surface of the polishing table is preferably restricted to 15 xcexcm or less. The surface displacement in rotation of the polishing surface of the polishing table means displacement of the polishing surface of the polishing table in polishing action in an almost vertical direction at any position on the polishing surface.
Rotational displacement in rotation of a rotary shaft of the polishing plate is preferably restricted to 30 xcexcm or less. The rotational displacement in rotation of the rotary shaft of the polishing plate means displacement in an almost horizontal direction at any position of the rotary shaft of the polishing table in polishing action. Note that requirements for the rotational unevenness of the polishing table, the surface displacement in rotation of the polishing surface of the polishing table and the rotational displacement of the rotary shaft of the polishing table can all be met by improving precision of a rotation system of the polishing table.
Furthermore, it is preferable that the work holding plate has recesses or a rib structure formed on a rear surface thereof By thus forming the recesses or the rib structure on the rear surface thereof like the polishing table, the work holding plate becomes lighter in weight while retaining its strength, and the recesses can be utilized as a path for a temperature adjusting fluid.
As described heretofore, in the polishing apparatus, the work holding plate not only supports a work physically, but also operates as an important factor to achieve the object of the present invention, and it is especially important to suppress the deformation thereof during polishing action. For this reason, it is preferable that as structural materials thereof, ceramics materials, among them alumina or silicon carbide (abbreviated as SiC) is used taking account of values of a mechanical strength and a thermal conductivity, workability, adhesiveness against a wafer and further, cost performance as well.
Moreover, as a method for holding a wafer on a work holding plate in addition to a method using an adhesive, a method of vacuum chucking a wafer on the work holding surface of the work holding plate is employed; therefore a structure is useful that a plurality of fine holes for vacuum chucking a work are opened in a region of the work holding plate where the wafer is adhered.
According to a first aspect of a polishing method of the present invention, there is provided a polishing method using a polishing apparatus with a polishing table and a work holding plate, wherein a work held on the work holding plate is polished, and in polishing action, an amount of deformation of the polishing table in a direction normal to an upper surface thereof and/or an amount of deformation of the work holding plate in a direction normal to a work holding surface thereof is restricted to 100 xcexcm or less. It is more preferable that the amount of deformation is restricted to 30 xcexcm or less.
According to a second aspect of a polishing method of the present invention, there is provided a polishing method using a polishing apparatus with a polishing table and a work holding plate, wherein a work held on the work holding plate is polished supplying a polishing agent solution, and when a to-be-polished surface of the work is polished by a polishing cloth adhered on the polishing table, temperature changes at any position on a polishing surface of the polishing cloth in polishing action are controlled to 10xc2x0 C. or less. The temperature changes are preferably controlled to 5xc2x0 C. or less.
According to a third aspect of a polishing method of the present invention, there is provided a polishing method using a polishing apparatus with a polishing table and a work holding plate, wherein a work held on the work holding plate is polished supplying a polishing agent solution, and temperature changes of the work in polishing operation are restricted to 10xc2x0 C. or less. The temperature changes are preferably controlled to 5xc2x0 C. or less.
It is an important embodiment of the present invention that temperature changes at any position on the polishing surface of the polishing cloth and/or temperature changes of a wafer in the polishing action are controlled to 10xc2x0 C. or less, preferably to 5xc2x0 C. or less by controlling a temperature and/or a flow rate of the polishing agent solution.
According to a fourth aspect of a polishing method of the present invention, there is provided a polishing method using a polishing apparatus with a polishing table and a work holding plate, wherein a plurality of works held on the work holding plate are polished, and the plurality of wafers are arranged and held on the work holding plate so as to satisfy a relationship expressed by the following formula (1) with errors within 2 mm:
R={(r+x)+sin(xcfx80/N)(r+2y)}/sin(xcfx80/N)xe2x80x83xe2x80x83(1)
(in the above formula (1), R: a diameter of a work holding plate (mm), r: a diameter of a wafer (mm). x: a distance between two adjacent wafers (mm), y: a distance between a wafer and a peripheral edge of the work holding plate (mm), N: the number of wafers per work holding plate and xcfx80 : the ratio of the circumference to its diameter. The distance x between two adjacent wafers is measured at the mutually closest points on the respective peripheries.)
In the case where a plurality of wafers are held on one work holding plate, a way of arranging the plurality of wafers on a holding surface is very important. That is, it is important that the held wafers are polished under the same condition microscopically as far as possible, in other words, that the greatest possible even polishing conditions including a polishing rate are fulfilled between wafers and within a to-be-polished surface of one wafer. For that purpose, important factors are temperature on the to-be-polished surface, a pressure onto the polishing cloth, a method for supplying the polishing agent solution, a relative movement distance between the wafer and the polishing cloth and others, and the above-described formula has been obtained by collectively and empirically studying such factors.
In the case where the above described formula (1) is applied to a wafer of 200 mm or more in diameter, that is, r is 200 mm or more, it is required that 5xe2x89xa6Nxe2x89xa67, 5xe2x89xa6xxe2x89xa620 and 7xe2x89xa6yxe2x89xa622.
When a diameter (r) of a wafer increases to amount to 300 mm or more, a diameter (R) of a work holding plate increases as a matter of course. In company therewith, in order to suppress mechanical deformation, thermal deformation caused by temperature changes and others to the prescribed values or less, it is necessary that the thickness (d) of the work holding plate is increased with increase in the diameter (R), and as a result of various studies, in order to achieve the fundamental conception of the present invention that the amount of deformation of the work holding plate in a direction normal to the work holding surface thereof is restricted to 100 xcexcm or less, preferably 30 xcexcm or less, it is preferable that a thickness d of the work holding plate is determined such that aR less than d less than bR (a=0.04 to 0.08 and b=0.10 to 0.12).
According to a fifth aspect of a polishing method of the present invention, there is provided a polishing method, wherein a silicon wafer is polished using the polishing apparatus of the present invention described above.
In the polishing method of the third aspect described above, the polishing operation is preferably performed in an environment where temperature changes are restricted within xc2x12xc2x0 C. That is, in order to realize such high precision polishing, it is preferable that changes in an environmental temperature in a working space surrounding the polishing apparatus are restricted within xc2x12xc2x0 C. of the prescribed temperature.
There are important a way to hold the work (wafer) on the work holding plate and a precision of a holding state thereof, that is not only flatness of the work holding surface but also evenness of a space between the holding surface and a to-be-adhered surface of the wafer. Particularly, in the case where a wafer is adhered and held on the work holding plate using an adhesive, attention should be focused on residual air bubbles in an adhesive layer between the wafer and the work holding plate, bow of the wafer in adhesion, and a thickness of the adhesive layer and its evenness.
Therefore, according to a method for adhering a work of the present invention, there is provided a method, wherein a work holding plate with a plurality of fine holes opened in an adhering region thereof for vacuum chucking a wafer is used and the wafer is adhered with an adhesive on the work holding plate by evacuating air through the plurality of fine holes from the rear side of the work holding plate. Such a construction of the method makes possible to eliminate the defects of the prior art method described above, reduce a thickness of the adhesive layer between the wafer and the work holding plate and improve evenness of the thickness thereof.
At this time, in order to facilitate the adhering operation, an adhering temperature is preferably kept at normal temperature (20 to 30xc2x0 C.); in order to effectively perform the adhering, improve evenness of the thickness of the adhesive layer after the adhering (a deviation of the thickness is preferably 0.015 xcexcm or less for high precision wafer processing), and reduce residual air bubbles in the adhesive layer as far as possible, a viscosity of the adhesive is preferably adjusted in the range of 1 mPaxc2x7s to 10 mPaxc2x7s during the period from application of the adhesive until prior to adhesion.
In order to effectively remove heat generated in polishing by a temperature adjusting fluid of the work holding plate through a wafer, it is necessary to reduce thermal resistance due to the adhesive layer interposing between the wafer and the work holding plate to the lowest possible level; in order to suppress variations in thickness of the adhesive layer caused by elastic deformation of the adhesive, it is also necessary to regulate a thickness of the adhesive layer to 0.5 xcexcm or less, preferably 0.3 xcexcm or less on the average, and a deviation of the thickness desirably to 0.015 xcexcm or less.
A work holding plate of the present invention includes a plurality of suction holes for vacuum chucking a work in an adhering region on a work adhering surface of the work holding plate, each of the holes penetrating from the work adhesion surface of the work holding plate to a rear surface thereof.
By using the above described work holding plate of the present invention, the above described method for adhering a work of the present invention can be effectively performed.
Recesses or a rib structure is preferably provided on a rear surface of the above described work holding plate.
High precision wafer polishing finish becomes possible by polishing a silicon wafer which is adhered and held on a work holding plate by means of the above described method for adhering a work of the present invention. At this time, the use of the above described polishing apparatus is very effective for realizing high precision polishing finish implementing the fundamental concept of the present invention that in polishing action an amount of deformation of the polishing table in a direction normal to an upper surface thereof and/or an amount of deformation of the work holding plate in a direction normal to a work holding surface thereof is kept to be 100 xcexcm or less, preferably 30 xcexcm or less.